1. Field of the Invention
The present invention relates to a multi-value data recording and reproducing device such as an optical disk device that can be effectively utilized for recording and reproducing multi-value data on a recording medium such as an optical disk.
2. Description of the Related Art
A conventional optical recording and reproducing system is an optical disk system that uses a laser as a light source and performs recording and reproduction with bit strings, and includes a recording system and a reproducing system that are regarded as partial-response digital transmission paths. The recording system includes a modulating encoder that modulates original data to binary data, a recording equalizer that produces binary intermediate data to restrict error transmission between the recording system and the reproducing system, and a semiconductor laser driving circuit. The reproducing system includes a read amplifier, a waveform equalizer that distinguishes the binary intermediate data from multi-value signals read in synchronization with the recording equalizer, an equalizer controlling device that controls the tap coefficient of the waveform equalizer so as to automatically optimize the characteristics of the waveform equalizer, and a demodulating encoder that reproduces the original data from the distinguished binary intermediate data. Japanese Laid-Open Patent Application No. 2-312018 discloses an example of the above optical recording and reproducing system that records and reproduces multi-value signals.
In such an optical recording and reproducing system, the recording equalizer can be formed by a modulo adder circuit, and the waveform equalizer can be formed by a modulo adder circuit and a multi-value level determining system that has a tap variable in accordance with the equalizer controlling device. The multi-level determining system of the waveform equalizer can be formed by a reproducing waveform equalizing filter having a variable tap system, and a multi-value level identifier.
Also, information recording methods have been suggested to record multi-value information on the information tracks of an optical information recording medium. In those methods, the-recording of the multi-value information is performed through various combinations of the lengths of information pits in the tracking direction and the shifting amounts of the information pits with respect to the reproducing optical spot in the tracking direction. Further, information reproducing devices in compliance with the methods have been suggested. Each of those information reproducing devices includes a multi-division photodetector, a storage unit, and an information identifier. In such an information reproducing device, the multi-division photodetector detects reproducing light flux reflected from an optical information recording medium or penetrated through an optical information recording medium. The storage unit stores all the light quantities and light distributions corresponding to the information pits of multi-value information represented by the combinations of the lengths of predetermined information pits in the tracking direction and the locations of the predetermined information pits with respect to the reproducing optical spot. The information identifier identifies the information of each information pit through correlations between the light quantities and light distributions stored in the storage unit. Japanese Laid-Open Patent Application No. 5-128530 discloses an example of the above type of information reproducing device.
However, there are several problems with the above optical recording and reproducing system, the above information recording method, and the above information reproducing device.
The above optical recording and reproducing system utilizes a multi-value level determining system and a modulo adder circuit as a waveform equalizer. As shown in FIG. 17, the multi-value level determining system and the modulo adder circuit have taps variable in accordance with the equalizer controlling device, so that interference among codes can be eliminated and information can be reproduced with high precision. In this circuit structure, waveform equalization is performed as a linear operation (a linear function) on input signals.
FIG. 18 illustrates an example of a multi-value recording operation in which the area occupancy rates of recording marks are varied with respect to the unit areas called “cells”. This operation will be hereinafter referred to as the “area modulation”.
With each recording mark, the reflectance becomes lower than the reflectance in unrecorded areas (i.e., “High-to-Low recording”). As shown in FIG. 18, the signal output values (prior to waveform equalization) of a reproducing signal in areas in which recording marks do not exist in the cells are indicated by marks ● numbered {circle around (1)}, {circle around (2)}, and {circle around (3)}. Even if the occupancy rates of the recording marks in the cells are the same, there are differences among the signal output values due to the difference between the occupancy rates of each neighboring recording marks, as indicated by {circle around (1)}, {circle around (2)}, and {circle around (3)}. This is because the relationship between the diameter DM of the recording and reproducing spot and the cell length (the period of time during which the recording and reproducing spot performs scanning in the direction of the arrow shown in FIG. 18) is DM>CL. Thus, the differences among the signal output values can be regarded as interference among codes.
Referring now to FIGS. 19A through 21B, the relationship between the interference among codes and the occupancy rates of each neighboring recording marks will be described.
FIGS. 19A and 19B illustrate a situation in which recording has not been performed in the cells located in front of and behind a subject cell (this situation will be hereinafter referred to as the “solitary wave” situation). FIGS. 20A and 20B illustrate a situation in which the cells located in front of and behind the subject cell each has the same recording mark occupancy rate as the recording mark occupancy rate of the subject cell (this situation will be hereinafter referred to as the “continuous wave” situation). FIGS. 21A and 21B illustrate a situation in which both of the cells located in front of and behind the subject cell have the highest recording mark occupancy rate.
The multi-value levels (0 through 7) in the graphs of FIGS. 19A, 20A, and 21A, indicate the recording mark occupancy rates. More specifically, the multi-value level “0” indicates an unrecorded state of a cell, and the multi-value level “7” indicates the state of a cell having the highest recording mark occupancy rate. Here, the multi-value recording is octal recording. Also, each of the graphs has ● marks representing the values measured prior to a waveform equalizing operation, and has a solid line representing target values. Each of the target values is a calculated value that represents a situation in which waveform interference can be completely eliminated when a waveform equalizing operation is performed with the circuit shown in FIG. 17.
As shown in FIG. 19A, the difference between the measured values and the target values exhibits a linearly proportional relationship (a linear relationship) with respect to the respective multi-value levels, and can be corrected by altering equalizing coefficients (equivalent to the constants C0 through C4 in FIG. 17) for linear operations through waveform equalization.
As for FIG. 20A, the measured values are substantially the same as the target values, and thus, interference among codes can be eliminated through waveform equalization.
On the other hand, as shown in FIG. 21A, the difference between the measured values and the target values is large in the area of the multi-levels 0 through 2, and does not exhibit a linearly proportional relationship with respect to the multi-value levels. This proves that waveform interference cannot be completely eliminated by the circuit shown in FIG. 17. As described above, in a case where waveform interference contains components that do not have linear influence (for example, a case of the multi-value recording by the “area modulation” technique described with reference to FIG. 18), there is a problem that interference cannot be adequately eliminated by waveform equalization.
To solve the problem that interference cannot be completely eliminated by waveform equalization by the above information recording method and the information reproducing device in a case where the interference among codes contains components having no linear influence (for example, the multi-value recording by the “area modulation” described with reference to FIG. 18), the influence of waveform interference is learned in advance through all the combination patterns, correction is added to the waveform equalizing operation, and a neural network that imitates the human information processing mechanism is utilized as a means of minimizing each waveform equalizing error in the learning process.
However, a considerable period of time is required for determining the convergence conditions for minimizing the error with each recording mark. As a result, in a case where an unknown data area is reproduced by reproducing a learning area inserted in a data area, the data reproducing speed cannot be increased though the reliability in data reproduction can be increased.